Mat films consisting of an inflexible hydrophobic support and an overlying mat layer containing a matting agent such as silicon dioxide, titanium oxide or glass powder are directly used for drawing purposes or coated with a photosensitive emulsion layer to form a wash-off photographic material which is exposed in a selected image area, developed and has the non-image area subsequently washed off.
In order to be used for these purposes, mat films are especially required to have the following features:
(1) They have high tensile strength, tear strength and bending strength, no tendency of curling and are dimensionally stable against changes in temperature and humidity;
(2) letters can be easily written into the mat films in drawing ink or with a pencil and they can be easily erased from the mats;
(3) letters can be rewritten in the retouched area in drawing ink or with a pencil a number of times.
Various approaches to improve erasability of letters from mat films have been considered. For example, Japanese Patent Application (OPI) No. 53067/81 (corresponding to U.S. Pat. No. 4,366,239) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses that such an improvement is achieved by incorporating poly(methyl methacrylate) and nitrocellulose in the mat layer as binders. Other conventional binders for the mat layer include vinylidene chloride copolymers (see, for example, British Pat. No. 1,047,697 (U.S. Pat. No. 3,370,951)); poly(methyl methacrylate) (see, for example, U.S. Pat. No. 3,353,958); polyesters or polyester amides (see, for example, U.S. Pat. No. 3,627,563); acetyl cellulose (see, for example, Japanese Patent Publication No. 48844/74 (U.S. Pat. No. 3,615,554)); and cellulose esters and polyesters (see, for example, Japanese Patent Publication No. 39414/74). After letters written in drawing ink on the mat layer containing these binders were erased with a rubber eraser, frequently, it was not possible to write lines of the desired fineness in the erased area because it repelled the drawing ink.